Mosquito carries, kills malaria parasite

JOHNS HOPKINS (US) — Some mosquitoes are a double-edged sword for the parasite that causes malaria, hosting not only the parasite itself but also a microbe that kills it.

The bacterium, known as Enterobacter, produces free radical oxygen-containing molecules that stop the parasite’s development. The findings could lead to new approaches for controlling malaria, which afflicts more than 225 million people and annually kills nearly 800,000, many in Africa.

Many parasites (blue) are found in blood that has not been exposed to the bacterium (A), but almost no parasites can be found in blood that contain the bacteria (blue rods) (B). (Credit: George Dimopoulos)

“We’ve previously shown that the mosquito’s midgut bacteria can activate its immune system and thereby indirectly limit the development of the malaria parasite,” says George Dimopoulos associate professor of molecular microbiology and immunology at Johns Hopkins University.

“In this study, we show that certain bacteria can directly block the malaria parasite’s development through the production of free radicals that are detrimental to Plasmodium in the mosquito gut.”

The parasite that causes the disease, Plasmodium falciparum, lives harmlessly in the gut of mosquitoes but infects and sickens humans when the host mosquitoes feed on human blood.

The discovery may explain why mosquitoes of the same species and strain differ in their resistance to the parasite. Individual wild mosquitoes may or may not carry Enterobacter in their guts, so they may differ in their ability to spread the parasite to humans.

“We may also use this knowledge to develop novel methods to stop the spread of malaria,” Dimopoulos says. “One biocontrol strategy may, for example, rely on the exposure of mosquitoes in the field to this natural bacterium, resulting in resistance to the malaria parasite.”

Like humans, mosquitoes have a variety of bacteria in their digestive system. For the study, the researchers isolated the Enterobacter bacterium from the midgut of Anopheles mosquitoes collected at Macha in southern Zambia.

About 25 percent of the mosquitoes collected contained the specific bacteria strain. Laboratory studies showed the bacterium inhibited the growth of Plasmodium up to 99 percent, both in the mosquito gut and in a test tube culture of the human malaria parasite. Higher doses of bacteria had a greater impact on Plasmodium growth.

The research was supported in part by the National Institutes of Health/National Institute of Allergy and Infectious Disease.